On 9/6/06, Tamas K Papp
or does the compiler perform this optimization? More generally, if a function is invoked with the same parameters again (and it doesn't involve anything like monads), does does it makes sense (performancewise) to "store" the result somewhere?
I was wondering something like this too, and then I found this email: http://www.haskell.org/pipermail/glasgow-haskell-bugs/2004-December/004530.h... So I guess it is as Stephane said: theoretically possible but not actually done? --eric the perpetual newbie The trouble is that this isn't always an optimisation. Try these two programs: powerset [] = [[]] powerset (x:xs) = powerset xs++map (x:) (powerset xs) and powerset [] = [[]] powerset (x:xs) = pxs++map (x:) pxs where pxs = powerset xs Try computing length (powerset [1..n]) with each definition. For small n, the second is faster. As n gets larger, the second gets slower and slower, but the first keeps chugging along. The problem is that the second has exponentially higher peak memory requirements than the first. Round about n=25, on my machine, all other programs stop responding while the second one runs. You don't really want a compiler to make that kind of "pessimisation" to your program, which is why it's a deliberate decision to leave most CSE to the programmer. You can still write the second version, and suffer the consequences, but at least you know it's your own fault! John